22 replies to this topic

[CHEMSTRONG]

Sir,

 

I have Ethylene Plant Charged Gas Compressor System for Steady State Process Simulation in some preliminary engineering software. I need to develop CGC flowsheet in simulator and it will be steady state model only.

 

I have Feed Flow in MTPH, Feed Mole Fractions for all feed components ,etc. How can I convert these component mole fractions to mole flows for all component. I need to do this as my simulation software does not accept mole fractions for components and instead it accepts only component mole flows Kmol/hr etc.

 

MY present approach is first evaluate average molecular weight of entire feed by multiplying individual component mole fractions to its component molecular weight and then taking sumproduct to get average molecular weight.

I use this final average molecular weight and  divide total flow of feed in MTPH by this molecular weight  and then i get feed molar flow here.

I use this feed molar flow and I have comp mole fraction already and then what i do is 

 

: Individual component mole fraction multiplied by Total Feed molar flows to get individual mole flows

 

is this correct?

 

Please help me and guide me in this.

 

Best Regards,

Chetan Chavan 

[breizh]

Hi,

It sounds good to me !

Breizh 

[Pilesar]

What simulation software makes it so difficult to characterize a process stream? Try entering the component mole fractions as mole flows and see if also entering the mass flow rate will automatically scale the molar rates. If it won't scale, then it should at least give you a resulting mass rate for the stream to allow you to ratio using the desired mass flow rate. If that does not work and you also have access to a commercial process simulator, enter the data there and allow the user-friendly simulator to calculate the component molar flow rates for you. Then copy the result into your software. Better yet, do the whole simulation in a commercial simulator and then try to duplicate it in 'your' software.

Edited by Pilesar, 18 December 2021 - 06:56 AM.

[CHEMSTRONG]

Sir,I am using Chemcad software and have some excel tool which is connected to Chemcad. I need to send feed composition in mole flows to Chemcad as Chemcad will not accept mole fractions. Now I have main issue is how to convert feed mole fractions into mole flows for all components ?? So I can not follow your earlier advice of copy pasting date from other simulator as user can not do everytime there is a change in flows

[CHEMSTRONG]

Also Excel based sheet is connected to Chemcad software for all Feed conditions but There is a limitation on Chemcad side that it can not accept Mole fractions from Excel it can only accept mole flows for all components. With my calculation I m facing an issue as I send my calculated mole flows to excel , my total flow in kg is getting changed a lot

[latexman]

Yes, your approach in Post #1 sounds good to me too.

[Bobby Strain]

If you had presented your query with equations it would be mush easier to respond. And, maybe the problem lies in your Excel spreadsheet, not ChemCad. You can modify the spreadsheet to input stream data in  component fractions. Look carefully at Chemcad to see what is really available. And modify the spreadsheet to suit your needs.

 

Bobby

[CHEMSTRONG]

I checked my excel spreadsheet,

I use simple formula like Average Molecular weight of mixture and use this value to get feed moles as I have feed mass rate in kg/ hr.
I divide feed mass rate by average mole weight to get feed moles

I am facing following issues now

1) I have configured Individual component mole fractions to evaluate Average Molecular Weight of feed gas stream.i have some doubt on some molecular weight as I took them from web e g Propadiene mole cular weight is 178 for me. Also what can molecular weight for c6-c8 fractions in Ethylene compressor feed. Also What is molecular weight of fuel oil fractions ?
2) when I send all these calculated moles of component to Chemcad to Chemcad feed stream, it is showing little bit higher feed mass rate as Chemcad calculates it based on sent moles
How to correct this error ?

[Bobby Strain]

You should show the composition. Then someone might get you closer. You should know the formula for propadiene.

 

Bobby

[CHEMSTRONG]

Dear Sir,

Ethylene feed composition is basically hydrogen, methane, ethylene, propylene, and heavier fractions like c6-c8 , Fuel Oil fractions etc. I have also detailed feed composition for all components but still do not have breakup of Those heavier fractions

[CHEMSTRONG]

Dear Sir,

Ethylene feed composition is basically hydrogen, methane, ethylene, propylene, and heavier fractions like c6-c8 , Fuel Oil fractions etc. I have also detailed feed composition for all components but still do not have breakup of Those heavier fractions

[breizh]

Hi ,

Propadiene  : C3H4   MW is about 40g/mol 

 

Consifder to review the MW of all your components , very basic stuff . 

 

Breizh 

[Pilesar]

Do you already have a converged and verified solution in Chemcad including the compressor intermediate flash drums? If so, you already have the characteristics you need for the components. If you do not have a complete simulator solution, get that portion done before messing with the Excel input.

[CHEMSTRONG]

Hi Breizh/ Pilesar,

Thanks for your support firstly.

I will explain my issue in more details for everyone to understand.

I have Methyl Acetylene and Propadiene MAPD as A component. For which I gave considered mol weight as 80.

For some components , thet are not pure components and their definition is not complete in all senses. E g C6 to C8 mixture components in Ethylene Plant feed , then they call it fuel oil cut components in Ethylene Plant feed. For these components, how to estimate the molecular weight ? ?

Simulator can not do that for me. If I do not consider it accurately, my feed moles calculation will go wrong.

Issue is I am operating Steady State CHEMCAD Centrifugal Compressor from Excel based User Interface and Limitation on Chemcad side is that when it is integrated with Excel, model feed stream can only accept Component Moles flows and NO Weight fractions or Volume fractions or Mole Fractions also.

I need to define Feed stream from Excel based UI, but I can not send mole fractions as feed composition. But all my input data is in mole fractions only here.

So I need to calculate feed components molar rates in Excel only before sending it to CHEMCAD . I have used formulas to evaluate this. But i think it is not accurate.

In this excel formula ,I need your advice about how to make accurate and verify its accuracy also



Best Regards,
Chetan Chavan



Best Regards,
Chetan Chavan

[breizh]

Hi ,

So the only way is to perform a full chemical analysis of your stream . Find internal resource or ask for 3rd party .

Good luck

Breizh 

[Bobby Strain]

You are too concerned about detail. You can define MAPD as propadiene. C6-C8 is mostly aromatics. So, pick a few and weight them to the lighter components. Pick something like naphthalene for fuel oil. That will get you close enough. That's what engineers do. Good luck.

 

Bobby

[Pilesar]

Bobby's answer is the way to do this. Remember that this is a compressor simulation with multiple inter-stage separation drums. The liquid knocked out at high pressure is sent back to a lower pressure flash drum so there are inherent recycles in the process. The heavier components are knocked out and removed from the system at the lower pressure stages. After you get a working simulation model, check the sensitivity of your component assumptions by characterizing C6-C8 as 'all hexene' and then 'all octene' to see how much difference it would make. 

[CHEMSTRONG]

Respected Members 

I am confused about one point now

CHEMCAD excel integrated UI :

I need to define feed composition of Process model from excel 

But CHEMCAD only accepts mole flows for component, Temp and Pressure of feed stream and NO Feed flow in kg/hr

CHEMCAD calculates the flow itself on its own.

But issue is now 

When I send  mole flows kmole/hr to CHEMCAD , it overestimates flow in kg/hr and this results in miscalculation downstream of process model as flow caclculated by CHEMCAD is around 206 kg/hr and my real flow is 200 kg/hr

how to solve this issue 

[Bobby Strain]

I wouldn't worry about 3% error. And this seems to be only an academic exercise with such small flow. You keep asking the same questions, but we have no idea of the reality.

 

Bobby

[CHEMSTRONG]

Dear Sir, But this error magnifies for higher flow . How to correct this

[Pilesar]

Test the software. Send over streams with single components only. Is the problem with all components? Is it just one or two? Is the molecular weight different? Find the problem and then devise a workaround. It might be as simple as using a hidden 0.97 factor in the spreadsheet for every stream flow. Commercial software is not free of bugs. Features are released even though some problems are known to the developers. Always test your results when you can since you cannot rely on software quality to be perfect. 

[CHEMSTRONG]

Dear Sir, my actual feed flow is day 200 mtph and I am sending feed component moles to software and then software evaluates its own feed flow which is around 206 .
This means that software takes lower molecular weight of components to evaluate final feed flow as compared what I have taken in Excel sheet ???

[Bobby Strain]

Your numbers don't make sense.

 

Bobby